how do i troubleshoot p0300 code.seems like cylinders 2,4,6,8 are not firing on 2003 Chevrolet Express 2500

have an engine code of p0300 multiple cyl misfire, put new wires ,plugs, fuel filter, and checked coil packs for each cylinder, all have spark, checked injector plugs for power all have power,. when first started engine runs smooth, after approx a minute it starts to shake and rumble, when its put into gear gets even worse.if i pull plug wires off of the cylinders on the passenger side any one of them the engine makes no changes. what else do i look for?